Getting Started with Blink Debugging

Introduction

While many of the tools and tips on this page can be used for it, this page focuses on debugging Blink outside the context of the web tests. For more web test specific instructions, see this page. For more general Chromium debugging info, see the respective pages for debugging on Windows, Mac, and Linux.

Linux

Getting Started

There are two main ways to get into blink: via debugging the chromium binary itself or content_shell. For most purposes of exclusive Blink debugging, the latter is the recommended option because it drastically reduces size and complexity. This means building content_shell, which should be as simple as making it the build target for your build method of choice. This should stick a content_shell binary in your out/Default directory.

content_shell itself takes as an argument the HTML file you wish to run Blink on. Furthermore, one of the simplest types of debugging you might want to do is to see the basic page structure after a page load (this internal structure in Blink is called the Layout Tree, not to be confused with the DOM Tree or the Line Box Tree). You can do this with a simple command line option of --run-web-tests. Thus, one of your simplest debugging tools, seeing the page structure after a page load, might look something like:

content_shell --run-web-tests test.html

Starting the Debugger

Debugging on Linux is generally done with GDB, so we will assume that's what you are using here. Not surprisingly, you will almost always want to compile Blink in debug mode to get all the symbols and tools you will need.

You will also want to use the gdbinit script to help gdb find source code, pretty-print common types, and other such niceties that will avoid you staring at assembly code. See docs/gdbinit.md for more details.

Important: Previous revisions of this document suggested running with the --single-process when debugging. That flag is no longer supported; it may work, it may fail outright, and it may appear to work yet manifest subtle differences in behavior that will cause you to waste many hours in understanding them.

You may need to modify your system's ptrace security policy before you can debug the renderer (details); run this command:

echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope

Both content_shell and chromium spawn renderer sub-processes; to debug blink, you need to attach a debugger to one of those sub-processes. The simplest way to do this is with the --no-sandbox and --renderer-startup-dialog parameters:

out/Default/content_shell --no-sandbox --renderer-startup-dialog test.html

When you do this, content_shell will print a message like this:

[10506:10506:0904/174115:2537132352130:ERROR:child_process.cc(131)] Renderer (10506) paused waiting for debugger to attach. Send SIGUSR1 to unpause.

This tells you that the pid of the renderer process is 10506. You can now attach to it:

gdb -p 10506

When gdb loads up, set whatever breakpoints you want in the blink code and 'continue'; for example:

(gdb) b blink::LayoutView::layout
(gdb) c

Then, send SIGUSR1 to the renderer process to tell it to proceed:

(gdb) signal SIGUSR1

If you are running web test, third_party/blink/tools/debug_web_tests does almost everything except for SIGUSR1 automatically.

If you see 'Could not find DWO CU' errors, you may need to have a symlink to the build directory from the current working directory.

% cd third_party/blink
% ln -s ../../out/Default/obj .
% ./tools/debug_web_tests --target=Default

General Useful Debugging Tools

Debugging functions

There are some key functions built into objects once you've reach a breakpoint inside Blink. For the examples here, we'll assume you're using GDB on Linux. These can be incredibly useful for showing the trees midway during execution to try and identify points when things change. You can use the GDB command print to display them. Here are some of Blink's debugging functions:

Assuming a local variable child in scope that's a LayoutObject, the following will print the Layout Tree:

(gdb) print child->showLayerTreeForThis()

#### Blink GDB python library

When using a GDB build that supports python, there's a library of useful Blink functions and pretty printers that can make working with some of Blink's types easier and more convenient, such as pretty printers for LayoutUnit and LayoutSize classes. You can find it at third_party/blink/tools/gdb/blink.py; see [LinuxDebugging](https://chromium.googlesource.com/chromium/src/+/HEAD/docs/linux/debugging.md) for instructions.

Printing back trace

Use Chromium's StackTrace

#include "base/debug/stack_trace.h"
...
base::debug::StackTrace().Print();
// or
LOG(ERROR) << base::debug::StackTrace();

and run Chrome with --no-sandbox command line option.

Debugging Printing related issues

It is difficult to understand whether an issue lies in the Print Preview logic or the Rendering logic. This document uses some of the tools available to us to help solve this issue.